Wind turbines have received increased attention as a renewable energy source. Wind turbines use the wind to generate electricity. The wind turns multiple blades connected to a rotor. The spin of the blades caused by the wind spins a shaft of the rotor, which connects to a generator that generates electricity. Certain wind turbine systems include a doubly fed induction generator (DFIG) to convert wind energy into electrical power suitable for output to an electrical grid. DFIGs are typically connected to a converter that regulates the flow of electrical power between the DFIG and the grid. More particularly, the converter allows the wind turbine to output electrical power at the grid frequency regardless of the rotational speed of the wind turbine blades.
Increased power DFIG systems can use a converter system that includes multiple bridge circuits connected in parallel for each output phase of the DFIG. When multiple bridge circuits are connected in parallel, the current sharing between the parallel bridge circuits during transients, such as at turn-on and at turn-off, does not balance. A number of factors can lead to this imbalance. For example, driver circuits used to drive the switching devices (e.g. insulated gate bipolar transistors (IGBTs)) used in the bridge circuits can contain opto-couplers for isolation of control signals. Each of these opto-couplers can provide different delay times in the control signals. Different delay times in the control signals can cause differences in the switching times of the switching devices (e.g. IGBTs) used in the bridge circuits. Any difference in timing between switching of the switching devices can cause a voltage across an inductor coupled to an output of the converter, leading to a circulating current between the parallel bridge circuits.
The imbalance in current can result in a difference of temperatures in the switching devices used in the parallel bridge circuits, such as a difference in junction temperature of IGBTs used in the switching modules. This reduces the overall output power capability of the converter as the total output current capability will be limited by the switching device with the highest temperature.
Thus, a need exists for a system and method of current balance control across parallel bridge circuits in a converter used for a DFIG wind turbine system. A system and method that reduces current imbalance across the parallel bridge circuits would be particularly useful.